The conventional optical circulator is a non-reciprocal multi-port routing and isolation component used in optical communications systems. FIG. 1 illustrates the operation of a generalized conventional four-port optical circulator 100. Light that enters the circulator 100 at port A 102 exits the optical circulator 100 at port B 104. However, light that enters the conventional optical circulator 200 at port B 104 does not travel to port A 102 but instead exits at port C 106. Similarly, light entering the port C 106 exits only at port D 108, and light entering port D 108 exits only at port A 102. In general, given a set of n equivalent optical input/output ports comprising a certain logical sequence within an optical circulator, light inputted to any port is outputted from the logical next port in the sequence and is prevented from being output from any other port. Since a light signal will travel only one way through any two consecutive ports of the optical circulator 100, such ports, in effect, comprise an optical isolator. By installing a reflector at one port of a generalized n-port optical circulator (where n.gtoreq.4) such that light outputted from the port is reflected back into the same port, the circulator may then be utilized as an (n-1)-port circulator. Furthermore, by blocking or failing to utilize one port of a generalized n-port optical circulator (where n.gtoreq.4), the device may be used as an (n-1)-port quasi-circulator.
The main application of optical circulators is in bi-directional optical fiber communications whereby two signals at the same wavelength may simultaneously propagate in opposite directions through a single fiber. In this way, optical circulators permit a doubling of the bit carrying capacity of an existing unidirectional fiber optic communication link since optical circulators can permit full duplex communication on a single fiber optic link.
FIG. 2 shows the basic components of a conventional optical circulator. The optical circulator comprises two polarization beam splitters 202 and 204, two 45-degree Faraday rotators 206 and 208, two half-wave plates 210 and 212, two mirrors 214 and 216, and four fiber optic input and output ports 218, 220, 222, and 224. The two Faraday rotators 206 and 208 rotate the polarization plane of linearly polarized light 45 degrees in one direction (for instance clockwise) as viewed from a fixed reference point (for instance, the left side of FIG. 2), regardless of the direction of light input. The two half wave plates 210 and 212 also rotate polarized light 45 degrees, but the direction of rotation is constant (for instance clockwise) as viewed from the side at which light enters the plate. Signal light input comprising unpolarized light may be input from any one of the four ports 218-224 into either one of the two polarization beam splitters 202 or 204, which separate the light into two linearly polarized sub-signals, one p-polarized and the other s-polarized. These sub-signals propagate through the other optical elements. By inspection, it may be verified that light input at Port A 218 is transmitted to Port B 220, light input from Port B 220 is transmitted to Port C 222, light input from Port C 222 is transmitted to Port D 224, and light input from Port D 224 is transmitted to Port A 218. Thus, the circulator 200 is a 4-port optical circulator.
Other conventional circulator designs employ numerous stacked optical elements, such as waveplates, Faraday rotators and polarization beam splitters and optical input/output ports optically coupled to the stacked optics and disposed not all to one side of the apparatus. Such conventional arrangements are bulky and complex and cause difficulties for optical alignment.
Accordingly, there exists a need for an improved optical circulator. The improved optical circulator should minimize the number of required optical elements and should be easier to align than conventional optical circulators. The present invention addresses such a need.